Dispersion flattened fiber with high negative dispersion and method for the manufacture thereof

ABSTRACT

The invention relates to a dispersion flattened fiber (DFF) with high negative dispersion and a manufacturing method thereof. The dispersion flattened fiber comprises a central core; ring-type cores and low refractive regions alternately formed outside the central core; a cladding surrounding outside the ring-type cores and low refractive regions; and a coating outside the cladding. Since the dispersion flattened fiber has the dispersion of −20 to −60, it has a wide range of application and can be used for various purposes in the field of optical telecommunication.

FIELD OF THE INVENTION

The present invention relates to a dispersion flattened fiber (DFF) withhigh negative dispersion and a method for the manufacture thereof; and,more particularly, to a dispersion flattened fiber with high negativedispersion to be utilized for a dispersion compensation in aconventional single mode fiber (SMF) or a non-zero dispersion shiftedfiber (NZDSF) by setting up a dispersion thereof to be high negative,i.e., ranging, e.g., from −20 to −60, not zero, at a wavelength band of1.55 μm.

DESCRIPTION OF THE PRIOR ART

In the field of optical communications, dispersion is defined as apulse-spreading phenomenon caused due to the fact that the wave velocityof an optical signal passing through an optical fiber varies dependingon the wavelength thereof.

As a conventional optical fiber for transmission, there exist an SMFoptimized for a 1.31 wavelength band and an NZDSF with a smalldispersion for 1.55 μm wavelength band, and the like.

However, when the conventional SMF or NZDSF is used, a maximumtransmission distance is limited as a transmission speed increases.Generally, the relationship between the transmission speed B [Gb/s] andthe maximum transmission distance L is shown as follows: $\begin{matrix}{L = \frac{104000}{B^{2} \times D}} & {{Eq}.\quad 1}\end{matrix}$

wherein D represents a dispersion.

When the SMF (whose dispersion is about 17 ps/nm/km at a wavelength of1.55 μm) is used to transmit an optical signal at a speed of 2.5 Gb/s, amaximum transmission distance is 979 km according to Eq. 1, but when thetransmission speed increases to 10 Gb/s, the maximum transmissiondistance is diminished to just about 60 km. When the NZDSF (a dispersionthereof is about 2 to 7 ps/nm/km) is used to transmit the optical signalwith the transmission speed of 10 Gb/s, the maximum transmissiondistance is limited to about 148 km. In the case of adopting awavelength division multiplexing (WDM) transmission method that featureshigh speed and big capacity, a dispersion slope as well as thedispersion must be taken into consideration in order to estimate amaximum transmission distance.

Accordingly, in order to increase the maximum transmission distance at apredetermined wavelength band, it is essential to compensate not onlythe dispersion but also the dispersion slope. As a solution to thisassignment, a dispersion compensation fiber (DCF) has been developed.Although the DCF compensates for both the dispersion and the dispersionslope simultaneously, the manufacturing process thereof is toocomplicated.

Up to now, researches in the DCF have been mainly focused on a methodfor flattening the dispersion to be nearly zero at a wavelength band of1.55 μm.

When the SMF is employed to transmit an optical signal at a transmissionspeed of more than 10 Gbps, both the dispersion and dispersion slope,which limit directly the maximum transmission distance, must becompensated. This can be achieved by employing a DCF that compensatesboth the dispersion and the dispersion slope at the same time. However,a manufacture of the DCF has not been easy.

A variety of methods for compensating a dispersion of an optical fiberby using a dispersion compensation module, which comprises DCFs, havebeen developed. Since it is not easy to produce a DCF capable ofsimultaneously compensating both the dispersion and the dispersionslope, an alternative method using two separate DCFs for exactdispersion compensation has also been developed as follows:

L _(DCF1) ×D _(DCF1) +L _(DCF2) ×D _(DCF2) +L _(SMF) ×D _(SMF)=0  Eq.2

$\begin{matrix}{\frac{S_{SMF}}{D_{SMF}} = \frac{{L_{DCF1} \times S_{DCF1}} + {L_{DCF2} \times S_{DCF2}}}{{L_{DCF1} \times D_{DCF1}} + {L_{DCF2} \times D_{DCF2}}}} & {{Eq}.\quad 3}\end{matrix}$

wherein L _(DCF1), L _(DCF2) and L _(SMF) represent the maximumtransmission distance of a first DCF, a second DCF and a SMF,respectively; D_(DCF1), D_(DCF2) and D _(SMF) stand for the dispersionof the first DCF, the second DCF and the SMF, respectively; andS_(DCF1), S_(DCF2) and SMF represent the dispersion slope of the firstDCF, the second DCF and the SMF, respectively.

In the case of using two different DCFS, it is required to combine thedispersions and the dispersion slopes of the two DCFS, so that the exactcompensation for the dispersion becomes more difficult.

SUMMARY OF THE INVENTION

It is, therefore, the object of the present invention to provide adispersion flattened fiber having high negative dispersion as well asflat dispersion characteristic at a transmission wavelength band so asto compensate the dispersion with advanced facility and exactness, andalso provide a manufacturing method of such dispersion flattened fiber.

In accordance with a preferred embodiment of the present invention,there is provided a dispersion flattened fiber with high negativedispersion comprising:

a central core;

ring-type cores and low refractive regions alternately formed outsidethe central core;

a cladding formed surrounding the ring-type cores and the low refractiveregions; and

a coating formed outside the cladding so as to protect the central core,the ring-typed cores, the low refractive regions and the cladding.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of preferred embodimentsgiven with reference to the accompanying drawings in which:

FIG. 1 represents a cross sectional view of a dispersion flattened fiberwith high negative dispersion in accordance with a first embodiment ofthe present invention;

FIG. 2 is a schematic drawing showing a refractive index of the opticalfiber of FIG. 1 along its radius;

FIG. 3 is a graph showing a C-band characteristic of the optical fiberin FIG. 1;

FIG. 4 presents a graph illustrating an L-band characteristic of theoptical fiber as shown in FIG. 1;

FIG. 5 depicts a table describing a design and characteristics of theoptical fiber shown in FIG. 1; and

FIG. 6 sets forth a flow chart of a manufacturing method for thedispersion flattened fiber with high negative dispersion in accordancewith the first embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a cross sectional view for showing a structure of a dispersionflattened fiber with high negative dispersion in accordance with a firstembodiment of the present invention. The dispersion flattened fibercomprises a cladding 15, a first and a second ring-type core 14 and 12,a first and a second low refractive region 13 and 11, and a central core10.

At the outmost region of the dispersion flattened fiber, there is formeda polymer coating (not shown) to protect the dispersion flattened fiber.There is located the cladding 15 inside the polymer coating and thefirst ring-type core 14 in accordance with the present invention withinthe cladding 15. The first low refractive region 13 is formed at theinner region of the first ring-type core 14. Inside the first lowrefractive region 13 there is located the second ring-type core 12 and,the second low refractive region 11 is formed within the secondring-type core 12. Finally, at the central area within the second lowrefractive region 11, there is formed the central core 10.

The refractive indexes of the central core 10 and the second ring-typecore 12 are higher than those of the other regions. The refractive indexof the cladding 15 is equal to that of pure silica. The first and thesecond low refractive region 13 and 11 have lower refractive indexesthan the cladding 15. The refractive index of the second ring-type core12 is the same as that of the first ring-type core 14. The second lowrefractive region 11 has the same refractive index as the first lowrefractive region 13. Ge or P may be added to increase the refractiveindex of the central core 10 and the first and the second ring-type core12 and 14.

FIG. 2 is a schematic drawing for showing a refractive index profilealong the radius of the fiber, in which the central core 10 has thehighest refractive index and the first and the second ring-type core 12and 14 have lower refractive indexes than the central core 10. Althougha step-type refractive index profile has been used in FIG. 2, ahill-type or curved refractive index profile can be included as well.

FIG. 5 is a table for showing design data and characteristics of theoptical fiber shown in FIG. 1, wherein variation of the refractive indexalong the radius of the optical fiber is shown. FIG. 3 shows the C band(1.55 μm wavelength band) dispersion characteristic of the optical fiberhaving the features described in FIG. 5.

Small changes in the diameters of the first low refractive region 13 andthe first ring-type core 14 do not influence much on the dispersion andthe dispersion flattened characteristics of the optical fiber. Unlikemost of the conventional optical fibers, the dispersion flattened fiberof the present invention has a much better bend loss characteristic,e.g., about 0.0001102 dB/km. Further, the dispersion slope of thepresent invention is flatter than that of the conventional dispersionflattened fibers.

FIG. 4 is a graph presenting an L-band (1570 to 1620 nm) dispersioncharacteristic of the optical fiber as shown in FIG. 1. The L-band isrequired for the high-density wavelength division multiplexing mode.

FIG. 6 is a flow chart illustrating a step-by-step process formanufacturing the dispersion flattened fiber with high negativedispersion through a modified chemical vapor deposition in accordancewith the first embodiment of the present invention.

First, in step S2 silica tubes are arranged exactly on a MCVD board.

The silica tubes are heated in step S4 by an oxygen/hydrogen burner at atemperature of 1900° C. to get rid of any impurities inside and outsidethe silica tubes.

In step S6, the cladding 15 is formed to prevent an invasion of OHradicals by using SiCl₄ to make the refractive index of the cladding 15identical with that of the silica tubes.

In step S8, GeCl₄ or POCl₃ is used together with SiCl₄ to form the firstring-type core 14 whose refractive index is higher than that of thesilica tubes within the cladding 15.

In step S10, the first low refractive region 13 whose refractive indexis lower than that of the silica tubes is formed inside the firstring-type core 14 by keeping fluorine source, e.g., C₂F₆ or SiF₄,flowing into the silica tubes together with SiCl₄.

The second ring-type core 12 having a higher refractive index than thatof the silica tube is formed within the first low refractive region 13by using GeCl₄ or POCl₃ gas together with SiCl₄ in step S12.

In step S14, the second low refractive region 11 having a lowerrefractive index than that of the silica tube is formed within thesecond ring-type core 12 by having fluorine gas C₂F₂ or SiF₄ togetherwith SiCl₄ flow into the silica tube.

The central core 10 with the highest refractive index is formed withinthe second low refractive region 11 by providing both SiCl₄ and GeCl₄into the silica tube and heating them by the burner in step S16.

A preform of the optical fiber having the refractive index profile givenin accordance with the present invention is manufactured in step S18 byheating the silica tube using an oxygen/hydrogen burner under hightemperature of 2000° C. or beyond to completely infill remaining holeswithin the silica tube.

Over-cladding or jacketing process can be carried out in step S20 ifrequired, where a silica tube is jacketed on the preform of the opticalfiber.

From the preform of the optical fiber manufactured as recited above,optical fiber of 125 μm in diameter may be extracted with an opticalfiber take-out apparatus. During this process, the optical fiber goesthrough a first and a second coating, and finally gets the optical fiberof 250 μm in diameter in step S22.

In view of the foregoing, the dispersion flattened fiber of the presentinvention has a negative dispersion ranging from −20 to −60 at thewavelength band of about 1.55 μm and also has a dispersion slope muchflatter than those of conventional dispersion flattened fibers. Inaddition, the dispersion flattened fiber can be easily manufacturedbecause of its high flexibility on the diameter.

While the present invention has been described with respect to theparticular preferred embodiments, it will be apparent to those skilledin the art that various changes and modifications may be made withoutdeparting from the spirit and scope of the invention as defined in thefollowing claims.

What is claimed is:
 1. A dispersion flattened fiber with high negativedispersion ranging from about −20 ps/nm/km to about −60 ps/nm/kmcomprising: a central core; ring-type cores and low refractive regionsalternately formed outside the central core; a cladding formedsurrounding the ring-type cores and the low refractive regions; and acoating formed outside the cladding so as to protect the central core,the ring-typed cores, the low refractive regions and the cladding,wherein a first ring-type core is formed within the cladding, a firstlow refractive region within the first ring-type core, a secondring-type core within the first low refractive region and a second lowrefractive region within the second ring-type core, and wherein therespective refractive indexes of the first and second low refractiveregion are lower than that of the cladding.
 2. A dispersion flattenedfiber with high negative dispersion ranging from about −20 ps/nm/km toabout −60 ps/nm/km comprising: a central core; ring-type cores and lowrefractive regions alternately formed outside the central core; acladding formed surrounding the ring-type cores and the low refractiveregions; and a coating formed outside the cladding so as to protect thecentral core, the ring-typed cores, the low refractive regions and thecladding, wherein a first ring-type core is formed within the cladding,a first low refractive region within the first ring-type core, a secondring-type core within the first low refractive region and a second lowrefractive region within the second ring-type core, and wherein thefirst ring-type core has the same refractive index as the secondring-type core.
 3. A dispersion flattened fiber with high negativedispersion ranging from about −20 ps/nm/km to about −60 ps/nm/kmcomprising: a central core: ring-type cores and low refractive regionsalternately formed outside the central core; a cladding formedsurrounding the ring-type cores and the low refractive regions; and acoating formed outside the cladding so as to protect the central core,the ring-typed cores, the low refractive regions and the cladding,wherein a first ring-type core is formed within the cladding, a firstlow refractive region within the first ring-type core, a secondring-type core within the first low refractive region and a second lowrefractive region within the second ring-type core, and wherein thefirst low refractive region has the same refractive index as the secondlow refractive region.
 4. A dispersion flattened fiber with highnegative dispersion ranging from about −20 ps/nm/km to about −60ps/nm/km comprising: a central core; ring-type cores and low refractiveregions alternately formed outside the central core; a cladding formedsurrounding the ring-type cores and the low refractive regions; and acoating formed outside the cladding so as to protect the central core,the ring-typed cores, the low refractive regions and the cladding,wherein a first ring-type core is formed within the cladding, a firstlow refractive region within the first ring-type core, a secondring-type core within the first low refractive region and a second lowrefractive region within the second ring-type core, and whereinGermanium or P is added to the central core and the first and the secondring-type cores.
 5. The dispersion flatten fiber of claim 1, 2, 3 or 4,wherein the coating is a polymer coating.
 6. The dispersion flattenfiber of claim 1, 2, 3 or 4, wherein a refractive index of the centralcore is higher than those of the first and second ring-type core.
 7. Thedispersion flatten fiber of claim 1, 2, 3 or 4, wherein the cladding hasthe same refractive index as the genuine silica.